1. Field of the Invention
The present invention relates to a pinned layer in a spin valve sensor which has a cobalt iron niobium (CoFeNb) or cobalt iron niobium hafnium (CoFeNbHf) film for increasing the magnetic softness and reducing current shunting of the pinned layer and, more particularly, to a pinned layer wherein the cobalt iron niobium (CoFeNb) or cobalt iron niobium hafnium (CoFeNbHf) film is located between first and second cobalt based films.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic signal fields from a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning a magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers is minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered at the interfaces of the spacer layer with the pinned and free layer layers. When the magnetic moments of the pinned and free layer layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer rotates from a position parallel with respect to the magnetic moment of the pinned layer to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel (AP) pinned depending upon whether the pinned layer is one or more ferromagnetic films with a unidirectional magnetic moment or an antiparallel (AP) pinned layer structure wherein a pair of ferromagnetic AP pinned layers are separated by an AP coupling layer with magnetic moments of the AP pinned layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer or structure and a dual spin valve sensor employs two pinned layers or structures with the free layer located therebetween.
Because of the interfacing of the pinning layer and the pinned layer the pinned layer is exchange coupled to the pinning layer. A unidirectional orientation of the magnetic spins of the pinning layer pins the magnetic moment of the pinned layer in the same direction. The orientation of the magnetic spins of the pinning layer are set by applying heat close to or above a blocking temperature of the material of the pinning layer in the presence of a field that is directed perpendicular to the ABS. The blocking temperature is the temperature at which all of the magnetic spins of the pinning layer are free to rotate in response to an applied field. During the setting, the magnetic moment of the pinned layer is oriented parallel to the applied field and the magnetic spins of the pinning layer follow the orientation of the pinned layer. When the heat is reduced below the blocking temperature the magnetic spins of the pinning layer pin the orientation of the magnetic moment of the pinned layer. The pinning function is effective as long as the temperature remains substantially below the blocking temperature.
In the presence of some magnetic fields the magnetic moment of the pinned layer can be rotated antiparallel to the pinned direction. The question then is whether the magnetic moment of the pinned layer will return to the pinned direction when the magnetic field is relaxed. This depends upon the strength of the exchange coupling field between the pinning layer and the pinned layer and the magnetic stiffness of the pinned layer. A measure of the stiffness of the free layer is its easy axis coercivity HC or uniaxial anisotropy HK. The easy axis coercivity is the amount of field required to switch the orientation of the magnetic moment of the free layer 180xc2x0 along its easy axis while uniaxial anisotropy HK is the amount of field required to rotate the magnetic field 90xc2x0 from its easy axis. If the coercivity of the pinned layer exceeds the exchange coupling field between the pinning layer and the pinned layer the exchange coupling field will not be strong enough to bring the magnetic moment of the pinned layer back to the original pinned direction. Until the magnetic spins of the pinning layer are reset the read head is rendered inoperative.
A desirable material for a pinned layer is cobalt iron (CoFe). It has been found that when a cobalt iron (CoFe) pinned layer is exchange coupled to a nickel oxide (NiO) pinning layer that the cobalt iron (CoFe) pinned layer acts as a seed layer for promoting a desirable texture of layers formed thereon. The result is an increase in the magnetoresistive coefficient dr/R of the spin valve sensor. Unfortunately, however, cobalt iron (CoFe) is magnetically stiff as manifested by its coercivity and uniaxial anisotropy. Accordingly, there is a strong felt need to decrease the magnetic stiffness of a cobalt iron (CoFe) pinned layer so that the exchange coupling field between the pinning layer and the cobalt iron (CoFe) pinned layer will return the magnetic moment of the cobalt iron (CoFe) pinned layer to its original orientation after being rotated therefrom.
The present invention provides a middle film composed of cobalt iron niobium (CoFeNb) or cobalt iron niobium hafnium (CoFeNbHf) which is located between first and second films composed of a cobalt based material which is preferably cobalt iron (CoFe). The magnetic stiffness of such a pinned layer is significantly less than a single cobalt based layer and is more responsive to magnetic spins of a pinning layer for returning a magnetic moment of the pinned layer to its original orientation perpendicular to the ABS when it is rotated therefrom by some extraneous magnetic field. As compared to nickel iron (NiFe), which has been considered as a middle film in the pinned layer, cobalt iron niobium (CoFeNb) or cobalt iron niobium hafnium (CoFeNbHf) has a 20% higher magnetization than nickel iron (NiFe). This means that cobalt iron niobium (CoFeNb) or cobalt iron niobium hafnium (CoFeNbHf) is more magnetically soft than nickel iron (NiFe). Another important advantage is that the cobalt iron niobium (CoFeNb) or the cobalt iron niobium hafnium (CoFeNbHf) middle film has significantly less sense current shunting than a nickel iron (NiFe) middle film. The first cobalt based film interfaces the pinning layer and provides the aforementioned desirable textures for subsequent layers deposited thereon by acting as a seed layer. The second cobalt based film interfaces the spacer layer and further increases the magnetoresistive coefficient dr/R of the spin valve sensor. The niobium (Nb) in the middle film raises the resistance of the middle film by making it amorphous. The hafnium (Hf) is optionally employed for adjusting the magnetostriction of the pinned layer more negative to a desirable zero value. The preferred middle film is cobalt iron niobium hafnium (Co85Fe2Nb10Hf2-5). The present pinned layer can be a single pinned layer in either a top or a bottom spin valve sensor and or employed in one or both AP pinned layers in an AP pinned layer structure. In the preferred embodiment the spin valve sensor employs an AP pinned layer structure with the first AP pinned layer next to the pinning layer structure incorporating the present pinned layer.
An object of the present invention is provide a pinned layer which has first and second cobalt based films for increasing a magnetoresistive coefficient dr/R of a spin valve sensor and a middle film which has less magnetic stiffness than the first and second cobalt based films and less current shunting as compared to a nickel iron (NiFe) middle layer.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.